For this study, researchers analyzed breast milk samples from the mothers to see what microbes they contained. Breast milk was previously thought to be sterile, but it does contain bacteria, the authors wrote in the paper.

"The initial question was, what does the milk bacteria profile really look like? This has been relatively understudied," study co-author Dr. Meghan Azad, a research scientist at the Children's Hospital Research Institute of Manitoba, told INSIDER. "We saw that it was highly variable — milk from different mothers contained different types and combinations of bacteria. So then our second question was, why the difference?"

Using information they'd collected about the mothers, the researchers analyzed how different factors, like how their baby was delivered, feeding methods, and body mass index, might be related to the differences in their milk's bacteria.

Different feeding methods were linked with a different mix of bacteria in breast milk, according to the study

"The most consistent factor, after exploring many possibilities, was the method of feeding," Azad said. "So moms who only fed at the breast had a different mix of bacteria in their milk than moms who used a pump."

First, the researchers found that compared with milk from moms who breastfed directly, pumped milk had lower levels of bifidobacteria, which is common in the infant gut and "generally thought to be a good and helpful bacterium," Azad said.

Second, milk from moms who pumped had a higher abundance of "potential opportunistic pathogens," Azad said.

"These are microbes that in certain contexts could possibly lead to infection, but this depends on other factors like what other bacteria are present, and whether the individual is immunocompromised," Azad said. "So it is not the case that if your milk contains these bacteria, your baby will necessarily get sick. It's not that straightforward. But it is an interesting finding that deserves further investigation."

The study didn't show that pumped milk is bad

Azad added that the study "does not show that pumped milk is bad."

"Breast milk is beneficial for many reasons, and for some moms, pumping might be the only way they can provide breast milk to their babies for a variety of reasons," she told INSIDER. "So we certainly don't want to discourage pumping, but rather raise the question of what does this mean and what further research needs to be done."

Instead, she added, the findings raise questions about what the difference in bacteria means and what future research should be done to better understand it.

Previous research has shown that babies fed pumped milk can have different health outcomes

Azad and her research group previously published studies showing that different infant feeding methods are linked with different health outcomes for babies.

"In those studies ... we've seen that babies who were exclusively breastfed at the breast have the healthiest body weights and also the lowest risk of developing asthma when compared to babies who received only formula," Azad said. "Breastfed babies who received some pumped breastmilk were in between — meaning they had a higher risk than babies fed exclusively at the breast, but a lower risk than formula-fed babies."

Azad said there are many possibilities that could potentially explain these differences. One idea is that pumping milk might change its composition — after pumping, certain bioactive elements in the milk might degrade, or the pump itself might introduce new elements, she added. For this new study, the researchers simply honed in on one specific aspect of breast milk composition: its bacterial profile.

The new findings could provide one clue that may explain why her research group observed different health outcomes in babies who were fed in different ways, Azad explained.

For now, the authors of the paper wrote that more research is needed to replicate these findings in new groups of participants and investigate their implications for babies' health.

Future research can help improve nutrition for all babies

More research in this area could improve nutrition for all babies, Azad said. Marco Bello/Reuters Pictures

Azad said that further investigation on this subject can lead to helpful recommendations for parents.

"By understanding more about the difference between pumped milk and breastfeeding...we can empower all moms, regardless of how they choose to feed their babies. This might involve new recommendations about bottle feeding, cleaning breast pumps, or storing pumped milk, or new supplements for babies that can't be breastfed," she said. "These are all things that we'll learn by doing the research."

She added that the message from this new study is not for any parents to feel alarmed or guilty.

"We know that parents are already under a lot of stress," she said. "But I think the bigger picture is that this type of research is important to improve informed decision making and nutrition for all babies in the future."